<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		<script src="js/jquery.min.js"></script>
		<script type="text/javascript">
			$(function(){
				$('#btn').click(function(){
						//这里加个判断
						if($('.class').length==1){
							return
						}else{
							//你要干的事情
						}
				})
			})
		</script>
	</head>
	<body>
		<button id="btn">添加</button>
		<div id="box"></div>
	</body>
</html>
